There is much prior art dealing with the expansion of perlite. This is a variety of obsidian which contains over 2% combined water. Various methods of expansion are disclosed in the prior art. The commercial process involves heating the perlite ore to approximately 500.degree. F. and then dropping it into a flame which is directed upwardly. When the ore reaches a temperature somewhere between 1400.degree. F. and 1800.degree. F., it expands very rapidly and is blown upward and out of the furnace by the exhaust gases.
Obsidian involved in this invention is obtained from a limited area in northern California. Similar material may be found elsewhere. Its principal components are as follows.
76.7% silica, 13.0% alumina, 1.37% iron, 0.42% calcium, 4.07% sodium, 3.93% potassium, 56 ppm barium, 316 ppm manganese, 25 ppm strontium, 550 ppm titanium. The approximate water content is 0.14%.
When this obsidian is heated to a high temperature, its volume increases by a factor of 20 to 25. The expanded product differs from expanded perlite in that the latter contains mostly open pores and therefore absorbs water. The material involved in the present invention produces an expanded product containing mostly closed pores and therefore does not absorb water. An attempt was made to expand it by the method referred to above, but this attempt failed since this obsidian must be heated to a temperature of at least 1900.degree. F. before it will expand. Furthermore, it expands very slowly while the rock is going through the viscous state and could not be supported by the fire blast.
An attempt was made to expand this obsidian in a fluidized bed at 2000.degree. F. This process also failed since the obsidian pieces conglomerated while in the viscous state and large clinkers fell to the bottom of the furnace. Some success was obtained with a rotating kiln made of Inconel, the obsidian being fed into the upper end of a inclined kiln and a flame being introduced near the lower end. The expanded product did not stick to the Inconel and an output of 30 pounds/hour was achieved in an extended run of 36 hours. However, the Inconel kiln failed soon thereafter. The Inconel is too expensive for a large kiln and the operating temperature of 2400.degree. F. is too close to its operating limit of 2400.degree. F.
It was then decided to try a rotating kiln lined with firebrick. However, it was learned from tests of stationary furnaces and later in a rotating kiln that the expanding obsidian sticks to the brick. It was also found that the obsidian would stick to itself, thereby producing large clinkers instead of the free-flowing homogeneous product.